In hybrid electric vehicles (HEVs), the fuel vapor canister primarily adsorbs refueling vapors, as refueling and diurnal vapors are sealed within the fuel tank by a fuel tank isolation valve. An air intake system hydrocarbon (AIS HC) trap may capture hydrocarbons emitted by leaky injectors and/or from fuel that may puddle in the engine intake. The AIS HC trap may also capture uncombusted fuel that is trapped within the engine cylinders themselves. An AIS HC trap is required for vehicles to be classified as practically zero emissions vehicles (PZEVs).
The contents of the AIS HC trap may be purged to engine intake during engine operation by opening an intake throttle plate, thus directing fresh air through the trap and desorbing bound hydrocarbons for combustion. However, hybrid vehicles may operate for prolonged periods without combusting fuel, thus limiting opportunities to purge the fuel vapor canister and AIS HC trap for combustion. This may lead to an increase in bleed emissions if the vehicle is parked for an extended duration while one or both adsorption beds contain hydrocarbons. Alternatively, the engine may be forced on to purge the adsorption beds which decreases the fuel economy of the vehicle.
Other attempts to address AIS HC purging include desorbing hydrocarbons from the AIS HC to the fuel vapor canister by applying a vacuum downstream of the canister. One example approach is shown by Yang et al. in U.S. Patent Application 2014/0318506. Therein, vacuum stored in the fuel tank is applied to the engine intake during an engine-off condition, thus delivering uncombusted fuel to the canister. However, the inventors herein have recognized potential issues with such systems. As one example, the method depends on fuel tank vacuum, which may develop at certain points in a diurnal cycle when ambient temperature is low. If the vehicle is parked in the sun, the fuel tank may initially develop a positive pressure, thus nullifying the method at the time when hydrocarbon breakthrough is most likely.
In one example, the issues described above may be addressed by a method for a vehicle engine, wherein during a first condition, a vehicle controller is placed in a sleep mode following a vehicle-off event and then awoken following a duration, at which time contents of an air intake system hydrocarbon trap are purged to a fuel vapor canister by operating an electric motor to rotate the vehicle engine in a reverse direction. Rotating the vehicle engine in a reverse direction causes atmospheric air to enter an intake of the engine via an exhaust of the engine, desorbing hydrocarbons bound to the air intake system hydrocarbon trap. By porting the desorbed hydrocarbons to the fuel vapor canister, bleed emissions from the air intake system hydrocarbon trap may be reduced.
As one example, the first condition may include an indication to purge the air intake system hydrocarbon trap based on engine operating parameters during a drive cycle immediately preceding the vehicle-off event. The purge indication may be issued in response to a drive cycle wherein the engine was operated in a combustion mode for a duration followed by a duration of vehicle operation in a battery only mode. In this way, if the AIS HC trap is likely to have adsorbed hydrocarbons without an opportunity to be purged to the engine for combustion prior to turning the vehicle off, the trap may be purged during the vehicle-off duration, thus limiting bleed emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.